1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a developing device for forming an image with a two-ingredient type developer made up of toner grains and magnetic carrier grains and a process cartridge including the same.
2. Description of the Background Art
It is a common practice with an image forming apparatus to form a toner image by using a photoconductive element or image carrier provided with a photoconductive layer on its surface and a developing device. A two-ingredient type developer, made up of a toner and a magnetic carrier, is extensively applied to the developing device because it is feasible for color image formation. When the developer is frictionally charged in the developing device due to agitation, the resulting electrostatic charge causes the toner to electrostatically deposit on the carrier. The carrier, thus supporting the toner thereon, is magnetically deposited on a sleeve or developer carrier, which accommodates a stationary magnet roller therein, and is conveyed on the sleeve due to the rotation of the sleeve.
The magnet roller includes a main pole for development located at a position where the sleeve adjoins the photoconductive element. When the developer being conveyed approaches the main pole, a number of carrier grains included in the developer gather and form brush chains, or a magnet brush, along the magnetic lines of force of the main pole. In a magnet brush type of developing system, it is generally accepted that the carrier, which is dielectric, increases the field strength between the photoconductive element and the sleeve to thereby cause the toner to move from the carrier around the tips of the brush chains to a latent image formed on the photoconductive element.
In an image forming apparatus of the type developing a latent image by conveying a developer deposited on a sleeve to a developing zone where the sleeve faces a photoconductive element, a doctor or metering member is usually configured to face the circumference of the sleeve at a preselected gap. In the magnet brush type of developing device, the doctor meters the developer brought to the above gap or doctor gap by the sleeve for thereby regulating the amount of the developer to reach the developing zone.
As for toner grains for use in the magnet brush type of developing system, inorganic fine grains of silica or titanium oxide should preferably be selectively deposited on the surfaces of toner grains as an additive. Such an additive enhances the fluidity of the toner grains and therefore the dispersion and rapid charging of the toner grains when the toner grains are replenished, thereby contributing to the formation of high-quality images.
However, the problem with the developer is that heavy stresses acts on the developer due to a long time of mixing and agitation and the presence of the doctor. The stresses cause the additive to part from the toner grains or be buried in the same and bring about the separation of carrier coating layers as well as toner spent, rendering the amount of charge to deposit on the toner grains unstable and reducing the durability of the entire developer.
More specifically, the inorganic fine grains of silica, titanium oxide or similar additive deposited on the toner grains are susceptible to mechanical and thermal stresses and therefore apt to part from the toner grains or be buried in the same due to repeated agitation in the developing device. Therefore, stresses to act on the developer should be reduced in order to maintain the amount of charge to deposit on the toner grains stable and the durability of the developer. This is also true even when such an additive is not applied to the toner grains.
Today, to meet the increasing demand for the size reduction of a copier or similar image forming apparatus, the size of the developing device is, of course, decreasing. While the size of the developing device may be reduced if the diameter of the photoconductive element and that of the sleeve are reduced, it can also be reduced if the amount of the developer and therefore the size of a developer chamber for storing it is reduced. However, in the case where the amount of the developer is reduced, it is necessary to reduce the amount of the developer not deposited on the sleeve because the developer must be present in the developing zone in a constant amount at all times. In this case, therefore, most part of the developer is deposited on and conveyed by the sleeve at all times and, as a consequence, subject to heavier stress ascribable to the doctor.
Further, for high-speed printing, a force for feeding the developer to the developing zone must be strong enough to maintain high image density. This requirement cannot be met unless the linear velocities of the photoconductive element and sleeve and developer conveying speed are increased, aggravating stresses to act on the developer.
On the other hand, the life of the developing device is determined mainly by the deterioration of the developer, particularly a decrease in the charging ability of the carrier ascribable to repeated development. The charging ability of the carrier decreases because the components of the toner grains locally deposit on the carrier grains. As for oilless toner grains, in particular, wax is dispersed in the toner grains for providing them with a parting ability in the event of fixation. When such toner grains are subject to stress, the resulting heat causes the wax, which is of the same polarity as the toner grains, to exude out of the toner grains and form films on the carrier grains, preventing the carrier grains from charging the toner grains when contacting the toner grains. As a result, the overall amount of charge of the toner grains decreases and brings about toner scattering, background fog and other defects.
Further, the developing device must meet the demand for high image quality, including sharpness, tonality and low granularity, as well as the demand for a long life.
To insure stable, high image quality over a long time with the developing device involving heavy stresses, as stated above, a developing device configured to reduce the stress and long-life carrier grains capable of enhancing image quality have been proposed in various forms in the past, as will be described hereinafter.
Japanese Patent Laid-Open Publication No. 11-161007, for example, discloses a developing device in which a doctor or metering member, facing a sleeve at a preselected gap, is implemented by a magnetic plate configured to form a magnetic field between it and a magnet disposed in the sleeve. The edge of the magnetic plate, facing the sleeve, includes a surface that approaches the sleeve little by little toward the downstream side in the direction of rotation of the sleeve. Such a doctor, according to the above document, stably feeds an adequate amount of developer to the sleeve to thereby reduce stress to act on the developer while reducing a load on a motor assigned to the sleeve. However, the stress to act on the developer does not occur at the edge of the doctor, but occurs mainly in a developer layer intercepted by one major surface or back of the doctor, as will be described more specifically later. The document does not address to the stress occurring in the above developer layer intercepted by the doctor.
Japanese Patent Laid-Open Publication No. 5-35067 teaches a developing device in which a cylindrical, developer conveying member is positioned just upstream of a doctor or metering member and constantly rotated while being spaced from a sleeve by a preselected distance at all times. This document describes that the developer conveying member prevents a developer from being packed in a metering position and does not form a stationary developer layer, which will also be described specifically later, thereby insuring stable image formation free from irregular density. In this developing device, however, a zone where the developer is packed exists between the doctor and the developer conveying member as in a conventional developing device including two doctors or predoctors. It is therefore likely that the developer is packed between the doctor and the developer conveying member when, e.g., the fluidity of the developer varies due to aging or the varying environment, forming a stationary developer layer that deteriorates the developer. Further, the developing device is sophisticated in configuration and therefore high cost.
Japanese Patent Laid-Open Publication No. 9-146374 proposes to position a magnet roller for holding a developer at a position upstream of a doctor and facing a sleeve, thereby reducing stress to act on the developer. The magnet roller, however, increases the amount of the developer, which stays at the position upstream of the doctor, more than when the magnet roller is absent, so that more developer is subject to stress by being held in a developer layer upstream of the doctor. Further, it is likely that stress to act on the individual developer grains increases.
Japanese Patent Laid-Open Publication No. 2001-109266 discloses a method that conveys a desired amount of developer to a developing zone only with magnetic field generating means disposed in a sleeve, thereby obviating stress ascribable to a doctor. Although this method reduces a frictional force and other stresses to act on the developer, toner contained in the developer cannot be sufficiently charged and therefore fails to form a satisfactory image.
On the other hand, to reinforce carrier coating layers, Japanese Patent Laid-Open Publication No. 9-311504 proposes to form hardened coating layers, which are formed of phenol resin containing an amino radical, on the surfaces of spherical, compound core grains made up of ion oxide grain powder and hardened phenol resin. Further, the above document proposes a particular iron oxide grain content and a particular amino radical content. These configurations, according to the document, implement stable frictional charging and durability.
Japanese Patent Laid-Open Publication No. 9-311505 proposes hardened coating layers, which are formed of one or more of melamine resin, aniline resin and urea resin and phenol resin, on the surfaces of spherical, compound core grains for the purpose of implementing stable frictional charging and durability.
Japanese Patent No. 2,825,295 proposes to coat the surfaces of grains, which are formed of ferromagnetic fine grains and hardened phenol resin, with melamine resin, thereby producing magnetic carrier grains with high electric resistance and low bulk density. Also, Japanese Patent No. 2,905,563 implements such carrier grains by uniformly coating the surfaces of grains, which are formed of ferromagnetic fine grains and hardened phenol resin, with polyamide.
Japanese Patent Laid-Open Publication No. 5-273789 proposes to deposit an additive on carrier surfaces while Japanese Patent Laid-Open Publication No. 9-160304 proposes a coating layer containing conductive grains whose size is greater than the thickness of the coating layer. Japanese Patent Laid-Open publication No. 8-6307 proposes a carrier coating material whose major component is a benzoguanamine-n-butylalcohol-formaldehyde copolymer. Also, Japanese Patent No. 2,683,624 proposes a carrier coating layer implemented by crosslinked melamine resin and acrylic resin.
However, considering the current trend toward a lower melting point and a smaller grain size of a toner material that aggravate the adhesion of toner components to carrier surfaces, the prior art schemes described above are not satisfactory in the aspect of a margin as to the adhesion of toner components to carrier surfaces. It is therefore difficult to obviate background fog, toner scattering and other problems, which are ascribable to a decrease in the amount of charge due to aging and lower image quality.
Not only high image quality but also high durability and stability are required of a modern copier, printer or similar image forming apparatus. More specifically, it is necessary to protect image quality from the varying environment and to constantly implement stable images over a long period of time. For example, in the magnet brush type of developing system using the two-ingredient developer, it has been customary to stabilize image density with an alternating electric field that superposes an AC component on a DC voltage to thereby alternately generate an electric field, which biases toner toward a photoconductive element, and an electric field urging the toner toward a sleeve. A high developing ability particular to the alternating electric field insures sufficient solid-image density even when the charge distribution of toner is shifted due to aging. At the same time, there can be generated an electric field strong enough for toner to develop even on a halftone or similar pattern whose latent image is relatively shallow. Such a technology, having the above advantages, is often applied to a color image forming apparatus, among others. Of course, the above technology is optimally applicable even to a monochromatic copier for reducing granularity of a halftone image and forming a uniform solid image.
The alternating electric field, however, brings about discharge due to a local increase in electric field ascribable to the irregular density of the magnet brush in the developing region, particularly in deep portions of a latent image, causing an image to be lost in the form of a ring. Therefore, the resistance of the carrier for development is limited, i.e., it is difficult to use a carrier with low resistance. Furthermore, even when a carrier with medium or high resistance is used, local breakdown ascribable to irregular coating layers occurs and also brings about discharge. In this respect, even the uniformly of carrier coating layers and the resistance of the carrier cores, i.e., the material of the cores are limited.
In light of the above, Japanese Patent Laid-Open Publication No. 2000-29308 proposes a technology for freeing a halftone portion, which adjoins a sold portion, from blur to thereby insure high image quality at all times. In accordance with this technology, the slip efficiency η of a developer relative to the surface of a sleeve is so adjusted as to satisfy a relation:Mb−Ma≧70g/m2where Ma denotes the amount of the developer for a unit area, as measured on the sleeve moved away from a doctor or metering member, and Mb denotes the amount of the developer for a unit area on the sleeve in a developing zone.
Further, Japanese Patent Laid-Open Publication Nos. 7-121031 and 7-128982 each propose to position the peak flux density of a main pole for development at a position where a photoconductive element and a sleeve adjoin each other, and to position a pole of opposite polarity having peak flux density within 40° at the upstream side in the direction of rotation of the sleeve. With this configuration, the above document describes that the density of a magnet brush increases to 6/mm2 or above and produces an image free from roughness.
However, in Laid-Open Publication No. 2000-29308 mentioned above, the slip of the developer in the developing zone, i.e., the slip of carrier grains, supporting toner grains, is sometimes undesirable in the aspect of high image quality that should be maintained despite aging. For example, the slip brings about carrier deposition and carrier scattering. Carrier deposition occurs when electric restraint, holding magnetic carrier grains on the sleeve, and electric attraction, acting toward the photoconductive element and derived from a background potential determined by a background potential and bias for development, are brought out of balance. To increase the slip efficiency of the developer for increasing the amount of the developer in the developing zone means to reduce a margin as to the deposition of carrier grains, which originally should be magnetically restrained.
Further, if the slip efficiency and therefore the amount of the developer in the developing zone is excessively increased, then the developer is packed in the upstream and center portions of the developing zone. As a result, a magnet brush rises in the upstream portion and obstructs development that should originally be effected when the magnet brush contacts the photoconductive element. Also, the developer packed in the center portion scrapes off toner grains present on the photoconductive element by scavenging, lowering developing efficiency in the developing zone. As a consequence, the boundary portions of a halftone region around a solid image, particularly a boundary at the upstream side in the direction of development, is lost. As for developing efficiency, the moving speed of the developer right above the sleeve and that of the developer around the photoconductive element should be the same from the efficiency standpoint, so that increasing the slip efficiency η translates into lowering developing efficiency.
Laid-Open Publication Nos. 7-121031 and 7-128982 also mentioned earlier have a problem that the density of the developer or that of the magnet brush in the actual developing zone is determined by a gap for development, the curvature of the photoconductive element and that of the sleeve, i.e., the density of the magnet brush measured on a developing roller differs from the actual system. For example, when an image is formed by the magnet brush having the density of 6/mm2 or above, as measured on a developing roller, and by a large gap for development, roughness is conspicuous in the image.